Electrical feedthroughs used in downhole logging tools, logging while drilling (LWD), and measurement while drilling (MWD) tools, as well as any other electronic instruments capable of being used in a downhole environment, are subjected to a variety of harsh operating environments. These electrical feedthroughs may carry substantial amounts of power with electrical signals which may be of a few thousand volts and/or of a few hundred ampere electric currents. The electronic instruments within a downhole logging tool requires a hermetic type electrical feedthrough that is able to interconnect with surface instruments for providing power, control signal, data transmission, and the like. The electrical feedthroughs must survive in extreme hostile liquid environments, such as brine, oil and water base drilling mud, and fluids that may contain hydrogen sulfide, carbon dioxide, methane, and moisture, including pressures of up around 30,000 PSI and temperatures of ˜177 degrees Celsius which are commonly encountered in downhole environments.
Usually, downhole logging instruments known as logging sondes are lowered into boreholes to make, for example, formation evaluation measurements, and infer properties of the formation surrounding the borehole and the fluids (gas, oil, water, or a mixed multi-phase) in the formation. These downhole logging tools may be an acoustic/ultrasonic logging tool, a neutron or gamma-ray density tool, a formation identification tool for measuring the earth formations surrounding a borehole, such as in a hydrocarbon (e.g., oil, natural gas, etc.) well. Such downhole logging instruments may be used to make such measurements while the well is being drilled, which is referred to as logging-while-drilling (LWD) or measurement-while-drilling (MWD). The LWD or MWD techniques may allow corrective actions to be taken during the drilling processes if desired. For example, borehole information, if available, in real time may be used to make adjustments to mud weights to prevent formation damage and to improve well stability. In addition, real time formation log data may be used to direct a drill bit in the desired direction. Usually, a downhole logging tool has electrical conductors mounted on the tool housing in a tubular structure. The logging tool includes a metal housing and an electrical wireline. A bulkhead is coupled to a tool housing that includes a metal shell for protecting an electrical connecting pins assembly. The electrical connecting pin is coupled to the exterior wireline cable, and to the interior electronic circuits, and a dielectric sealing material is used to insulate the electrical transmissions from logging tool electronics to the wireline cable to surface power or data processing unit. The downhole logging tool may be required for an open-hole or a closed-hole service bypassing a wellhead. A wireline cable not only mechanically supports the downhole tool but also simultaneously provides electrical power to the tool and sends the measured data back to a surface data process unit. A wellbore may be filled with fluids that may contain certain amounts of water and moisture. The electronics inside the downhole tool housing require a hermetic type electrical feedthrough (singular or multi-pin) that interconnects with surface power or data processing unit for power and control of the signal transmissions, or for data transmissions. For a plurality of logging sondes based downhole tools, each individual logging sonde has at least two electrical feedthroughs as interconnects for power or control signal transmissions. The extremely harsh environment deployable electrical feedthrough package should not only survive the elevated downhole temperatures at 30,000 PSI pressure but also require high corrosion-resistance for ensuring long-term operation reliability.
A hermetically sealed electrical feedthrough package could protect the inside of logging or measurement electronics or instruments from extreme hostile liquid environments, however, the electrical resistivity of the dielectric sealing material may be not only be rapidly declined with elevated downhole temperature but also decreased when exposed to water or moisture, thereby potentially causing catastrophic downhole tool electric failures either by dielectric sealing material moisture absorption or by its hydrophilicity.
Therefore a need exists for novel systems and methods for determining the impact of moisture on dielectric sealing material of an electrical feedthrough package. There is also a need for an evaluation system and method to identify if an electrical feedthrough package has high moisture resistance under simulated downhole conditions. A further need exists, for an evaluation system and method to identify if the dielectric sealing material of a downhole electrical feedthrough package has high moisture resistance or hydrophobicity under water-based or moisture-rich oil-based wellbores. More specifically, a need exists for an evaluation system and method for measuring electrical insulation resistance of an electrical feedthrough in general, and moisture resistance from the downhole electrical feedthrough package in particular, for enabling downhole logging tools, LWD and MWD tools to have reliable operation in water-based or moisture-rich oil-based wellbores.